1. Field of the Invention
The present invention relates to a cellulose acylate film and a producing method thereof, especially to a cellulose acylate film and a producing method thereof which is used as a protective film for a polarized film constructing a polarizing filter and which is used for an optical compensation film, a liquid crystal display and the like.
2. Description Related to the Prior Art
A cellulose acylate film has transparency, adequate water vapor permeability and high mechanical strength. Dimensional resistance of the cellulose acylate film does not depend on the humidity and the temperature so much. Therefore the cellulose acylate film is widely used as a base of an optical material. As a producing method of the cellulose acylate film, there is a solution casting method, in which a dope prepared from a polymer and a solvent is cast with a casting die on a substrate, and dried in a drying process to form a film.
The cellulose acylate film is also used as a protective film for a polarizing filter constructing a liquid crystal display. The polarizing filter is usually constructed of a polarized film which is formed of polyvinylalcohol, and a cellulose acylate film which is adhered to one or both of surfaces of the polarized film. As the polyvinyl alcohol is hydrophilic polymer, the film surface of the polyvinyl alcohol film has a good hydrophilic nature. Otherwise, the surface of the cellulose acylate film has a hydrophobic nature. Accordingly, in order to adhere the polyvinyl alcohol film and the cellulose acylate film, the condition of the film surfaces of at least one of the polyvinylalcohol film and the cellulose acylate film is modified.
For surface modification of the film, the film surface of the cellulose acylate film is usually saponificated. As the main saponification treatment, there are a method of coating or spraying an alkali solution on the film surface, a method of dipping the cellulose acylate film into the alkali solution (see, for example Japanese Patent Laid-Open Publication No. 2001-318228).
Further, cellulose acylate is usually produced from a wood pulp, cotton linter or the like. In the usual method of producing the cellulose acetate, for example, the wood pulp or the cotton linter is treated with acetic acid or acetic anhydride. Thus cellulose triacetate in which degree of acylation is 3 is obtained. The acetic anhydride is used as an agent for esterification, and sulfuric acid is used as a catalyst thereby. The remaining sulfuric acid as the catalyst is effective for saponification and ripening of the obtained cellulose triacetate, so as to control the degree of acylation. Thus the objected cellulose acetate is obtained. In this case, calcium compounds and the like are added so as to neutral the excess of several sorts of acid.
Japanese Patent Laid-Open Publication No. 2002-192541 teaches that cellulose acylate film contains the several sorts of compounds in order to improve the plastication and to provide optical properties. The compounds are often necessary not only in the cellulose acylate film but in several sorts of the films for optical use, in order to make the quality higher.
However, in any methods for saponification treatment to modify the surface of the cellulose acylate film, production facilities therefor are extremely large and cannot be equipped continuously just after the solution casting process. Accordingly, the cellulose acylate film produced in the solution casting method is wound and thereafter set to the system for the saponification. Then the cellulose acylate film is unwound so as to perform the saponification. In this case, the number of the producing process becomes larger by one, which prevents the higher productivity and the lower cost. Further, the saponification often causes the wrinkle and unevenness of the film, and the transparency of the film becomes lower.
The hydrophilic nature of the above cellulose acylate depends on the degree (that is an averaged degree) of acylation. When the averaged degree of acylation is low, the number of hydroxyl group contained in the same mole number of cellulose acylate is large. Accordingly, the averaged degree of acylation is preferably low in order to make the hydrophilic nature higher. However, when the averaged degree of acylation is low, the water vapor permeability, dimensional resistance of the film, peelability and the like become lower, and the cellulose acylate is not adequate for practical use.
The many sorts of the compounds added to make the quality of the film higher show poor compatibility with cellulose acylate and the like. Therefore the large amount of them precipitate on the film surface or volatilize in the film forming processes. Further, some emission compounds adhere to parts, members and the like in the film forming processes, and adhere, drop and so on to the film. Thus the quality of the produced film becomes lower.